Generally in a data storage device where a recording surface of a recording medium is scanned with a head to read and write data, a servo pattern written to the recording medium is read by the head to carry out servo control (hereinafter, positioning control) for positioning the head as means to raise data recording density. In the positioning control, a position error signal (hereinafter abbreviated as PES) is generated based on the servo pattern read out by the head from the recording medium and the head position is recognized based on this PES.
Nowadays, magnetic disk drives using magnetic disks as recording media are being used widely as external storage devices of computers. In addition, magnetic disk drives are also becoming popular as recorders of motion picture data and sound data. Also in a magnetic storage device as represented by these magnetic disk drives, a servo pattern recorded on a magnetic recording medium is read out by a magnetic head to acquire a PES and the position of the magnetic head is recognized based on this PES to carry out positioning control of the magnetic head (for example, see Patent Document 1).
Incidentally, the characteristics of the PES are unique to each head, that is, each manufactured data storage device provided with a head has different PES characteristics. Therefore, it is necessary to adjust the control system of each data storage device in its manufacture process and the like so that the head position can accurately be grasped from the PES.
To accurately recognize the position and amount of displacement of the magnetic head from the PES gain, it is desirable that the change of the PES gain is proportional to the amount of displacement of the magnetic head. In reality, however, the relation between the change of the acquired PES gain and the amount of displacement of the magnetic head is discontinuous and non-linear due to the characteristics and error of the magnetic head. Therefore, adjusting compensation is made for the sensitivity (head gain) of the magnetic head and the nonlinear relation between the PES gain and the change of the amount of displacement of the magnetic head so that the change of the PES gain and the displacement of the magnetic head have a proportional relation.
[Patent Document]
Japanese Patent Laid-open No. 8-124136
As mentioned above, as long as positioning control uses a servo pattern recorded on a recording medium, the control system must initially be adjusted according to the characteristics of the head.
However, the MR (Magneto Resistive) head, which is being used widely to read magnetically recorded data, shows a phenomenon called instability in which the condition of the magnetic head changes. If this instability occurs, since the magnetic head's susceptibility to off-track changes, not only the PES gain in the same situation but also the non-linear relation between the change of the PES gain and the amount of displacement of the magnetic head become different from the characteristics for which initial adjustment was made. Accordingly, accurate positioning control is not possible for the magnetic head if the control system is adjusted only initially.